Many different configurations and constructions of boilers have become known for burning a fuel to produce heat and transfer a portion of the heat to water, in order to produce hot water or steam. The fuel may be No. 2 heating oil, kerosene, natural gas, propane, coal or wood, for example. The main body or boiler chassis of known “sectional boilers” is constructed of cast metal (e.g. cast iron or aluminum) “sections” that are assembled and connected together by pipe nipples and bolts or threaded rods that pass through all of the sections and clamp them together. Each section has a hollow interior space that communicates with the hollow interior spaces of the other sections when they are all connected together. The hollow spaces for the water-filled boiler jacket of the boiler. Of particular interest in the present application are especially cast iron sectional boilers fired by heating oil, but other metal types and fuel types are also pertinent. For example, instead of cast iron sections, the boiler may be constructed of sections of any metal material that have been formed and produced by casting, forging, powder forging, sintering, pressing, stamping, etc. The hot boiler water is used for domestic or commercial space heating, commercial or industrial process heating, and/or for producing hot domestic or commercial tap water via further heat exchange from the hot boiler water to potable tap waterthrough an additional heat exchanger. Nonetheless, the present application is not limited to such boilers, but rather may also apply to other known constructions and uses of boilers.
A conventional “old-technology” single-pass cast iron sectional boiler 1 is schematically illustrated in FIG. 1 of the drawings of the present application. The boiler 1 includes a fire box or combustion chamber 5, an oil burner unit 8 and a primary heat exchanger 10 of a water-filled jacket above the combustion chamber 5. Sometimes, the water-filled jacket also extends behind and on the sides of the combustion chamber 5, and sometimes also under the combustion chamber (called a wet-base boiler), to provide additional heat exchange surfaces. The water-filled jacket including the primary heat exchanger 10 is formed of several cast iron sections including a front section 10A, a back or rear section 10C, and one or more intermediate sections 10B1, 10B2, etc., which are connected together by pipe nipples, bolts and/or threaded rods that extend all the way through the sections from front to back, as well as optionally gaskets, seals and the like. The front section 10A and the back section 10C each have a specialized individual configuration, but the intermediate sections 10B1 and 10B2 all have the same standardized configuration, and more of these intermediate sections can be provided in order to construct a larger boiler providing a greater heating capacity. The example boiler 1 illustrated in FIG. 1 is known as a four-section boiler, because it includes the four sections 10A, 10B1, 10B2 and 10C. Combustion gas flue passages 11 are formed respectively between successive ones of the boiler sections 10. Namely, in the four-section boiler 1 shown in FIG. 1, a first flue passage 11A is formed between the sections 10A and 10B1, a second flue passage 11B is formed between the sections 10B1 and 10B2, and a third flue passage 11C is formed between the sections 10B2 and 10C. The cast iron sections are typically formed with heat exchange pins, vanes, or fins 18 provided on the surfaces thereof bounding the flue passages 11, in order to increase the surface area of the heat exchanger available for heat exchange between the hot flue gases 20 and the cast iron material, and then through the cast iron ultimately to the water 17 contained in the primary heat exchanger 10 of the water filled jacket.
The burner unit 8 includes a motor driving an oil pump and an air blower, an oil nozzle 9 through which oil is ejected under pressure from the pump, an electric ignition system for igniting the oil ejected from the nozzle, and an electronic control system. The nozzle 9 sprays the high pressure oil in a cone pattern of very fine atomized oil droplets, mixed into a flow of air provided by the air blower. The atomized mist of oil droplets is ignited to produce a combustion flame 19 in the combustion chamber 5. To protect the cast iron boiler jacket or body from the extreme high temperature of the combustion flame 19, and to keep the combustion hot (i.e. insulated from the cool water on the other side of the cast iron combustion chamber wall), the back wall of the combustion chamber 5 is typically covered or lined with a target wall 6, for example of refractory ceramic fiber (RCF) board such as Kaowool™ or Ceraboard™ RCF board. For the same reason, the bottom floor and/or side walls of the combustion chamber 5 are typically covered or lined with an insulation blanket 7 such as a flexible blanket of refractory ceramicfiber (RCF) such as Kaowool™ or Ceraboard™ blanket.
Under proper combustion conditions, the oil should be completely combusted within the combustion chamber 5, to produce hot flue gas 20, which passes upwardly through the primary heat exchanger 10. Namely a first hot gas flow 20A passes upwardly through the first flue passage 11A, a second hot gas flow 20B flows upwardly through the second flue passage 11B, and a third hot gas flow 20C passes upwardly through the third flue passage 11C. In the flue passages, the hot gas gives off some of their heat to the heat exchange pins 18 and the other surfaces of the cast iron sections 10A, 10B1, 10B2 and 10C, and through the cast iron to the water 17 contained in the sections, thereby heating the water. Thus, all of the combustion gas or flue gas 20 has flowed in a single pass through the primary heat exchanger 10, from the combustion chamber 5 upwardly once through the flue passages 11 of the primary heat exchanger 10. The somewhat cooler flue gases are then accumulated or collected in a flue collector chamber or exhaust manifold chamber 12 above the heat exchanger 10. The flue collector chamber 12 is formed and enclosed within a flue collector hood or clean out cover 13 mounted on top of the heat exchanger 10. From there, the collected gases are directed out of the breech or flue outlet 14 of the boiler 1 as exhaust flue gas 21, which is then discharged through a chimney, a power venter, or other exhaust flue arrangement. Also, to reduce heat losses from the boiler to the surrounding ambient environment, the cast iron sections 10 forming the boiler chassis or body are surrounded by a layer of insulation 15 such as fiberglass wool, which is further covered or encased in outer cover panels 16, for example of painted sheet metal.
To achieve maximum efficiency, it is of course desired to extract as much heat as possible from the hot flue gas 20 and transfer it to the water 17 in the heat exchanger 10, thereby cooling the hot flue gas 20 as much as possible, and thus achieving the lowest possible gross stack temperature or breech temperature of the exhaust flue gas 21 at the breech 14. However, it is conventionally taught that the temperature of the exhaust flue gas 21 at the breech of the boiler must remain above at least about 340° F. to avoid problems caused by condensation of water vapor and sulfur components out of the exhaust flue gas 21 in the chimney flue or the like. Namely, the hot flue gas 20 includes water vapor, sulfur, and other components of the heating oil and the combustion air provided through the oil burner unit 8, and at breech temperatures below about 340° F. these components can begin to condense in the chimney flue and form a corrosive acidic liquid condensate that can cause significant corrosive damage and failure of the flue and the cast iron boiler if this condensate drips back down into the boiler. Also, colder spots in the boiler might suffer condensation directly in the boiler. The actual dew point or condensing temperature of the oil combustion vapor is about 117° F., so it must be ensured that the combustion exhaust gases remain above about 120° F. everywhere in the boiler and through the entire length of the exhaust system such as a chimney, until exiting at the top of the chimney. To ensure a chimney top temperature above 120° F., it is typically recommended to maintain a breech temperature at the breech outlet of the boiler above about 335° F. or 340° F.
However, due to inherent inefficiencies in the conventional single-pass sectional boiler 1 shown in FIG. 1, such boilers generally do not achieve such low breech temperatures that condensation problems arise. To the contrary, such boilers typically exhibit an extremely high breech temperature, for example gross breech temperatures in a range from 400° F. to 500° F. or even higher. As a result, much of the heat value of the input heating oil “goes up the chimney” in the form of unnecessarily and excessively hot exhaust flue gas 21, because of less than optimal heat transfer from the hot flue gas 20 to the water 17 through the heat exchanger 10. The first major cause of such inefficiency of the heat transfer is the typical conventional “single-pass” design of the heat exchanger as mentioned above. Namely, the hot flue gas 20 makes only a single pass upwardly through the heat exchanger 10. The hot flue gas 20 divides into three hot gas flows 20A, 20B and 20C, which each respectively pass a single time upwardly through a single flue passage 11A, 11B or 11C respectively, before being collected in the upper flue collector chamber 12 and being exhausted as flue gas 21 out through the breech 14. Thus, the hot flue gas 20 has only a relatively short distance of travel through the heat exchanger 10, and thus only a relatively short residence time in the heat exchanger 10, during which the heat transfer can take place.
Secondly, the heat exchange is also inefficient due to the direction of flow of the hot gas 20 relative to the direction of flow and the temperature stratification of the water 17 in the heat exchanger 10. The hottest water 17 naturally convects to the top of the heat exchanger 10. Also, the cool water returns to the boiler (e.g. from space heating radiators or the like) to a boiler water return inlet 22 connected to the bottom of the boiler water jacket, and hot water is tapped from the boiler through a hot boiler water supply outlet 23 connected to the top of the boiler water jacket. Thus, the flow of water is generally upward through the boiler and especially the heat exchanger 10, with cooler water toward the bottom and hotter water toward the top. The flow direction of the hot gas 20 is also upward through the heat exchanger 10. The hot gas 20 cools as it passes upwardly through the heat exchange passages, and the water 17 heats as it passes upwardly though each heat exchanger section. As a result, the gas 20 near the top of each heat exchanger passage is at its coolest temperature, but the water 17 near the top of the water jacket is at its hottest temperature. The rate of heat transfer between this coolest gas and hottest water is thus relatively low and inefficient. Basically, the heat exchanger is configured as a parallel flow heat exchanger, with both the primary hot fluid (hot gas 20) and the secondary fluid to be heated (water 17) flowing in parallel in the same direction. It is known that such a parallel flow heat exchange configuration is less efficient than a counter-flow heat exchange configuration, in which the two fluids flow generally in opposite directions, for example that the hottest flue gas 20 would be adjacent to the hottest water 17 while the coolest flue gas would be adjacent to the coolest water.
To improve the heat transfer efficiency and thus the overall efficiency, newer boilers with a multi-pass configuration have been designed. In such multi-pass boilers, the flue passages are configured so that the hot flue gas must flow in a serpentine back-and-forth fashion through the primary heat exchanger, through several adjacent flue passages in series one after another rather than in parallel as in the single-pass boiler described above. In such multi-pass boilers, for example boilers manufactured by Burnham and by Biasi, the flue gases typically flow horizontally back-and-forth, namely from the front to the rear of the boiler in the combustion chamber, and then forwardly through one set of flue passages, and then again toward the rear through another set of flue passages, before being exhausted out through the breech at the rear. Such boilers are typically called a three-pass boiler, although there are actually only two passes through heat exchanger flue passages and one pass through the combustion chamber. Such multi-pass sectional boilers can be expanded by adding additional sections as described above, to increase the total length of the boiler. While the flue passages thereby get longer, the boiler remains a three-pass boiler, i.e. no additional back-and-forth passes are provided. Nonetheless, such multi-pass boilers exhibit a significantly higher AFUE (Annual Fuel Utilization Efficiency) in comparison to the older technology single-pass boilers described above and illustrated in FIG. 1.
It has also become known to construct a boiler, particularly a gas fired boiler, to include baffles in the flue passages within each cast iron section to create a serpentine flow passage through the boiler, for example as disclosed in U.S. Pat. No. 5,109,806 (Duggan et al.). While such a boiler only provides a single pass through each boiler section, and only a single pass of the hot flue gas through the heat exchanger, the passage through each section is longer due to the serpentine flow pattern created within that flue passage by the baffles. As another improvement, it is known to provide fins within the passages of heat exchanger sections of a boiler, in order to redirect and distribute the hot gas flow from the combustion chamber into each respective flue passage segment in the boiler, for example as disclosed in U.S. Pat. No. 7,669,535 (Moskwa et al.). It is also known from U.S. Pat. No. 5,311,843 (Stuart) to arrange water flow diverter baffles selectively in the water flow header of a gas-fired water heater in order to achieve a single-pass or multi-pass flow of the water through the water passages of the heat exchanger.
While the above improved features of new technology boilers and water heaters achieve an improved efficiency in comparison to the operating efficiency of existing older technology single-pass boilers, that is unfortunately not directly helpful to the owners of an older single-pass sectional boiler that is otherwise still serviceable and operating well, except for a relatively low efficiency. In view of today's ever-increasing costs of heating oil, there is a strong urge to achieve the greatest efficiency of a heating boiler. The owner of such an older inefficient but serviceable boiler is thus faced with the dilemma of continuing to operate the old inefficient boiler with a higher ongoing operating cost due to the higher consumption of heating oil, or to pay a substantial initial capital cost to replace the old inefficient boiler with a newer more-efficient multi-pass boiler in hopes of achieving a payback of the capital expense over the course of several years in view of reduced operating expenses due to reduced heating oil consumption. Especially in view of the present high cost of heating oil, there is a great demand for avoiding this dilemma, for example by improving the efficiency of the existing older single-pass boiler, at a relatively low cost, without having to completely replace the existing boiler with a new multi-pass boiler.
A low-cost solution to overcome the above dilemma has been provided by U.S. Pat. No. 9,618,232 (Brown), which discloses converting an existing single-pass boiler (such as illustrated in present FIG. 1) to operate in a multi-pass manner. This is achieved by installing a refractory ceramic fiber (RCF) target wall in the existing combustion chamber, and an RCF draft diverter in the upper flue collector chamber. Those RCF components redirect the flue gas to flow in a multi-pass manner through the heat exchanger, namely upwardly through the first/front flue passage, then downwardly through the second/middle flue passage, and then upwardly through the third/rear flue passage. While that conversion method has been shown to achieve significant improvements in boiler efficiency, at a very low cost compared to the cost of a new multi-pass boiler, the solution is only temporary and requires periodic (e.g. annual) maintenance, upkeep and/or replacement of the (essentially temporarily installed) RCF target wall and RCF draft diverter. A more-permanent solution to the above dilemma is desired. Also, while achieving a multi-pass flow of the flue gas through the existing heat exchanger of the boiler, the conversion according to U.S. Pat. No. 9,618,232 does not increase the actual size and surface area of the heat exchange surfaces of the water-filled jacket. It is still desired to achieve further increases in boiler efficiency, and to increase the surface area of the heat exchange surfaces of an existing boiler, with a relatively low-cost retrofit or upgrade of an existing single-pass boiler, without having to purchase an entirely new multi-pass boiler.
Another approach to reduce fuel consumption and slightly increase heat transfer efficiency in an existing boiler is to derate or underfire the burner unit at a lower oil supply rate than the specified burn rate for the boiler. This can usually be accomplished simply by exchanging the burner nozzle with a smaller or lower rated nozzle, e.g. having a smaller nozzle orifice. Thus, the burner will inject and burn oil at a lower rate (e.g. gallons per hour), and also produce less combustion heat energy than the boiler is rated for. The heat exchanger thus becomes effectively bigger in proportion to the produced heat energy, and therefore the heat exchange efficiency increases slightly. This approach is especially applicable if the boiler capacity was originally oversized for the heating demand, or if upgrades to the building's insulation, windows, doors, air-sealing efforts, or the like have reduced the heating demand of the building below the original design heat load. In such cases, derating the oil burner reduces the heat output capacity of the overall boiler system to better match the required heat load, while also achieving slightly improved efficiency. However, such derating of the burner does nothing to address the inherent inefficiency of a single-pass up-flow heat exchanger arrangement of the typical conventional sectional cast iron boiler as discussed above in connection with FIG. 1. The combustion gases still flow too-rapidly through a single pass through the heat exchanger, and much of the available heat energy is simply wasted in the exhaust gas exiting the boiler at a higher temperature than necessary. It is still desirable to further improve the heat transfer efficiency in order to further reduce the boiler breech temperature and stack temperature by extracting more heat from the hot flue gases and transferring that heat to the boiler water.